The present invention relates to a radiation therapy treatment planning system and method for planning radiation therapy treatment by calculating dose at the point of concern in a patient.
Radiation therapy is conducted in a manner shown in FIG. 1. As illustrated, X-ray 38 from a radiation source 30 is irradiated with an irradiation Field 36 onto a diseased part 34 of a patient 32. Before actually conducting X-ray irradiation, the dose which will be applied to the point of concern must be calculated. This calculation is however is not simple, because of Compton scattering.
As shown in FIG. 2, X-ray 38 having entered from the air 31 into a patient 32 causes Compton scattering, by which X-ray 38 causes emission of electron 40, losing energy in the same amount given to the electron 40, and generating scattered X-ray 42 having a longer wavelength and proceeding in a different direction. The scattering angle .beta. is not greater than 90.degree.. The dose is proportional to the number of ion pairs created by the passage of electrons through the region of concern.
For determining the dose in a region 44, shown in FIG. 3, various categories or classes, classified in accordance with the history from which the electrons have been generated, must be considered. The first class consists of the electron 40a due to the Compton scattering of incident X-ray 38a. The second class consists of the electron 40b due to Compton scattering off scattered X-ray 42 derived from incident X-ray 38b. The third class includes all other electrons, i.e., electrons, not illustrated, derived through more than two Compton scatterings.
Because the electrons are generated by Compton scatterings in the neighboring regions, consideration needs to be given to the contributions from respective neighboring points to the point off concern in determining the dose at the point of concern.
Conventional methods off calculating dose taking account of the contributions from the neighboring points includes the Monte Carlo method, the convolution method, the delta volume method, the equivalent tissue air ratio (ETAR) method, the power law method, the scatter air ratio (SAR) method, carlo tissue air ratio (RTAR) method. Some of these methods are summarized in James A. Purdy, Computer Applications in Radiation Therapy Treatment Planning, Radiation Medicine, Vol. 1, No. 2, pp. 161-173, 1983.
The Monte Carlo method or the like in which 3-dimensional calculation is conducted will produce accurate results, but takes too long a time for the calculation. In the ETAR method and the like in which the 2-dimensional calculation is conducted, the time required for the calculation is short, but the accuracy of calculation is relatively low, so that it sometimes is impossible to satisfy the accuracy requirement, such as the one prescribed by the recommendation by ICRU (International Commission on Radiation Units and Measurements) of restraining the error in the dose to be within .+-.5%.